Sheet metal and method for its production.



O. F. BURGESS.

SHEET METAL AND METHOD FOR ITS PRODUCTION. APPLICATION FILED MAR; 2|, 1913.

1,144,106; Patented June 22 1915,.

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SHEET METAL AND METHOD FOR ITS PRODUCTION.

nuance,

Specification of Letters Patent.

Patented June 22, 11915.

Application filed March 21, 1913. Serial N 0. 756,004.

ful Improvements in Sheet Metal and Methods for ItsProductiOmof which the following is a specification.

It is the object of the present invention to produce sheet iron or steel through the body of which are layers or strata of metal electronegative to the main body of iron or steel, or strata of" metal, more resistant to corrosion than is iron or Steel. These strata may consist of elements, such as nickel or copper, or they may consist of alloys which have appropriate electronegative or corrosion resistant properties, such for instance, as nickel-iron alloy. The protective action of these layers or strata in the main body of metal can be made clearer by considering the chemical and electrochemical actions ordinarily involved in the corrosion of sheet iron or steel. F

If a sample of iron be examined carefully after it has been subjectedto corroding action, it Wlll be found that the surface is not uniformly eaten away, but that the rusting has taken place locally, to form pits or cavities. If rusting took place uniformly over the entire sheet of metal, the corrosion would be far less harmful than is found in practice where the pitting action is largely responsible forrapid deterioration of sheet iron and sheet steel.

Attempts have been made to explain pitting On the basis of the electrolytic theory, the usual explanation being that it is due to local galvanic couples caused by localized or segregated impurities or non-uniformity in the physical condition of the metal, but experimental' facts seem to controvert this theory. Almost all of the segregated impurities which can be produced iniron and steel are electronegative, but almost all of the pits which can be found in a corroded sheet are in the form of cavities without the high peak in the center which ought to be found there, if the theory is correct. Viewing thisquestion, therefore, from the electrolytic theory, a segregated impurity to cause round cup-shaped pitting, must be electropositive to the normal iron body, that 1s, t must have a higher solution tension, making it anodic with respect to the surrounding cathodic material, so that in the presence of moisture or electrolyte, the flow of current would be from the an bdic materials, with resultant corrosion of that anodic material. a

In contradistinction to the above theory of pitting, I suggest the following as bemg more in accord with practical results.

Busting may start from segregated impurities or differences in mechanical structure due to working or similar causes, but

when once started, the rusting accelerates and results in the formation of tubercules or rust mounds and the deterioration of the metal takes place in the form of pits which grow progressively downward into the metal through electrolytic action, until the sheet is perforated and destroyed.

A typical rustformjation consists of iron carbonate ina brown powdery, but coherent mass. These masses are distributed over the surface of the iron sheet. The material is somewhat hygroscopic and by attracting and retaining moisture from the air, maintains under the rust spot, a condition favorable to continued corrosion. We will suppose this iron to be exposed to a moist, atmosphere or to river water. Under such conditions the oxygen of the air or the dissolved oxygen in the river water can get at the clear exposed iron more readily than it can get at the iron under the rust spot.

This access of the oxygen to the clean iron tends to regeneity. We have in these potentials, a

cause of electrolytic corrosion which far outweighs the electrolytic corrosion which may be set up by internal impurities, as fre- 'quently described. his a well known fact that incipient rusting may be retarded for a long time, but when once it starts, the subsequent rusting is rapid, and we have here an explanation for this phenomenon, in that incipient rusting is due mainly to internal heterogeneity, While the subsequent and accelerated action is due to the heterogeneity set up by the rust spots themselves. Purity, and homogeneity associated w th purity, are important factors in retard ng incipient rusting, but when once the rustlng starts, purity isovershadowed by external conditions and becomes of minor importance. In accordance with the present invention, the sheet of iron or steel is made up with interposed layers or strata of metal, either elemental or an alloy so constituted as to resist the downward growth of the pits. These strata are of such a material that when encountered by the progress of the pit, they will set up an electrochemical potential in opposition to that producing the pit, thereby neutralizing the voltage previously existing and arresting the corrosion under the rust spot.

Figure 1 illustrates a magnified cross-section of a sheet having strata of nickel embedded in the iron to serve in arresting the inward growth or development of rust spots, and the strata. of nickel are indicated by solid lines, the varying Width of which indicate to some extent the variations which may be observed in the nickel layers after the composite sheet has been rolled and worked. In Fig. 2 is a similar illustration of the same kind of composite sheet after annealing at 1200 C. Under this treatment the nickel has to some extent alloyed with the surrounding iron to form strata of nickeliron alloy.

Nickel is one of the best materials for use.

in making these interposed strata in the iron or steel sheet and will give effective protection even when the quantity present, amounts to no more than 2% of the total body of the sheet. By distributing the nickel in very thin strata parallel to the surface of the sheet, the rust pits cannot progress inward without striking the nickel barrier, beyond which theycannot Well ass.

The sheets of iron or steel having distributed through them the strata of nickel,

' copper or alloys, may be made up in different /ways, as hereinafter indicated.

In accordance with one way of making up the sheets, we may start with a sheet or plate of mild steel, using this as cathode for the electrodeposition of iron. After the iron deposition has taken place for several days and the cathode has been built up to the desired thickness, it may be taken out, rinsed and placed in a nickel plating bath and there run for about one hour at the ordinary nickel plating current densities. It is then rinsed and returned to the iron tank thus built up electrolytically may then be rolled down'by ordinary rolling mill operations and will yield sheets of metal containing the nickel still retained in distinct layers or laminations between the successive layers of iron. The nickel is, of course, in exceedingly thin layers and under low magnifications, appears like a white line.

In heating up the iron with the included layers of nickel to. the temperature necessary for'rolling, there is a possibility of an alloying action between the nickel and the iron. The temperature, time of heating and other conditions of rolling, determine how completely this alloying may take place.

I have determined that at 900 degrees 0., the nickel is not materially dissipated by alloying action. If the temperature is kept as low as possible during the rolling operation (between 1100 and 800 C.), the alloying will be at a minimum, and even though some alloying be permitted, it is easily possible to prevent the nickel from alloying so completely with the iron as to produce a nickel-iron alloy of less than 20 per cent. nickel. A 20 per cent. nickel alloy has been found to be exceedingly resistant to corrosion and when formed by alloying of the nickel and iron can serve in place of pure nickel as the barrier for stopping the pits. By this method of alloying during rolling, on even by subsequent additional heat treatment when desirable, it is possible to secure the maximum resisting power of *nickel without using a large quantity of this expensive metal. Thus the protective action of pure nickel or of the 20 per cent. nickel alloy may be secured without having more than 1 or 2 per cent. of nickel in the sheet, and there is the probability that even smaller quantities of nickel will be effective, when distributed as strata parallel to the surface of the sheet.

'With the sheet constructed as above doscrlbed, corrosion may start on the iron surface because of impurities in the iron or because of its heterogeneous character, due to mechanical strains set up in rolling and the like and this incipient corrosion will be slow 1 n starting, if the surface layer is of the high degree of purity now obtainable 1n the market. But after corrosion has once started, with formation of a rust spot will promptly accelerate under the rust spot alloy, laying bare a small area of that metal,

the electronegative action of the exposed nickel with respect to the iron will set up a counter-voltage, opposin and neutralizing the previously active voltage and the bottom of the pit will now become electronegative insteadof electropositive with respect to the exposed iron surface beyond the rust spot. Aside from its electronegative char-. acter the stratum of nickel or nickel-iron al- 10y embodies corrosion resisting properties which are beneficial and this is particularly the case with a 20 per cent. nickel-iron stratum, for this, as is Well known, is substantially non-rusting.

After a rust spot has developed a rust pit and after this pit has worked down through the surface stratum of iron until it encounters and is stopped by the layer of nickel or nickel-iron-alloy, corrosion is resisted at the bottom of the pit so long as the equilibrium of electrochemical voltages remains undisturbed, but if from any cause, the rust pit shouldin course oftime break through the nickel stratum into the underlyingiron stratum, its growth through that underlying stratum would be retarded through the presence in that immediate vicinity of the residue of the punctured nickel stratum and if the pit should progress far enough to strike the second nickel stratum, it will there again be held up for a long time or checked entirely.

It may be said that with ordinary sheet lron or steel, purity and homogeneity of the iron at the surface are important factors in retarding lnclpient-corrosion, but when cor: rosion once starts, its action is concentrated .in the formation of pits which travel in- Ward with constantly increasing speed, their own growth accelerating further growth, and on the contrary the sheets of the pres ent invention do not lend themselves to this concentrated attack, but resist or check the,

inward growth of any of the pits, scattering the corrosion and giving to the sheet as a whole, a greatly increased useful life. As a modification in the method of building up the composite sheets and with the idea particularly of reducing the cost, we may begin with'a mild steel or iron starting sheet, giving that a coat of nickel, then givingit an electrolytic coating of iron, then athin layer of nickel and then a final layer-of iron. These composite sheets can then be rolled down at suitable temperature and will yield a sheet composed largely of iron or steel but having the protective strata. of nickel.

As another way of building up the corronickel-plating sheets of iron or steel and then stacking them together in a heating furnace and rolling them together under Welding conditions; and to decrease the diiiiculty of welding, I may, if desired, give to each coat a thin electrolytic covering of iron over the nickel before the bars or sheets are stacked and passed through the rolls.

Other metals than nickel may be used in this same way, such for example as copper, silver and cobalt, and even lead. Bronzes and brasses may also be used, these being applied either electrolytically or by hot brazing methods. For instance, sheet ars may be dipped through a layer of flux into 'molten brass or bronze, thus becoming coated and the bars then may be stacked together with some flux between them when necessary, and then heated and rolled. This yields a sheet in which the iron or steel carries embedded strata of electronegative metal.

I am aware that variations in the process of producing the sheet and in the chemical and physical makeup of the final product may be made without departing from the spirit of the present invention. Through out the claims the term iron is used generic to alloys or mixtures in which ironpredominates.

cupying but a small percentage of thetotal thickness of the sheet, said last named metal being of a chemical composition resistant to the inward growth of rust pits.

4. Sheet iron having distributed therethrough between iron strata thin and substantially continuous strata, which are high in nickel and resistant to corrosion.

5. Sheet iron having distributed therethrough between iron strata thin strata of nickel-iron alloy.

6. Sheet iron rendered resistant to the pitting action of rusting by the presence throughout the iron sheet of embedded strata of metal containing at least 20per cent. nickel, said strata occupying but a small percentage of the total thickness of the sheet and forming barriers to arrest the inward growth of rust pits.

7. The method of producing corrosion resistant sheet iron which consists in building up a composite plate from superposed layers of nickel and layers of iron and then rolling said composite plate into the form of sheet metal.

8. The method which consists in building up a composite plate out of superposed lay- In witness whereof, I hereunto subscribe ers of iron and layers of nickel, the nickel my name to this specification in the preslayers occupying/but a small percentage of ence of two Witnesses.

the total thickness of the plate and then CHARLES F. BURGESS. 5 hot. rolling into the form of sheet metal at Witnesses:

a temperature of 1100 to 800 0., substan- WALTER B. SCHULTE,

tially as described. L. T. RICHARDSON. 

